Conventionally, solenoid valves are used for increasing or decreasing the brakefluid pressure in ABS control, automatic braking control, etc. The solenoid valves are known in the form of a valve having a valve hole switched between full closing and full opening by a plunger and a valve having the opening degree of a valve hole proportionally controlled by controlling a plunger at a middle position between full closing and full opening. Due to the control reliability, size reduction, cost reduction, etc., of the apparatus, the bake controllers typically use the former solenoid valve.
One of the known methods of controlling the valve hole at the middle opening degree as ensured in the latter valve by using a solenoid valve with two switching positions of full opening and full closing relies upon the use of pulse-width modulation control (hereafter refer to as PWM control)(see, for example, a Japanese document of JP-A 8-295225).
However, brake controllers for carrying out the above PWM control do not provide means for determining whether or not a desired brake-fluid pressure is formed by locating the plunger of solenoid valve in a desired position. Feedback as to whether or not the solenoid valve carries out a desired operation is ensured in accordance with an actual wheel velocity, such as, for example, whether or not the wheel velocity rises or lowers to a target value after implementation of brake-fluid pressure control.
In such a way, when brake-fluid pressure control was carried out based on feedback on the wheel velocity, a time was absolutely needed until the result of operation of the solenoid valve is shown on the wheel velocity, resulting in a problem that a time was needed until the wheel velocity converges at a desired slip ratio.
Therefore, an improvement in the control responsibility is demanded to shorten a time up to convergence of the wheel velocity, and thus to obtain improved control quality.
In connection with one means for solving this problem, the inventors of the application guessed that the control responsibility could be improved by detecting in which position the plunger of the solenoid valve is controlled with respect to a desired position, which is then feedback controlled.
A prior art technique for checking the position of the plunger of such solenoid valve switching two positions of full opening and full closing is known from means disclosed in a Japanese document of JP-A 5-211105.
This prior art technique is configured such that a coil of the solenoid valve is connected in series with a current detection resistance, and is supplied with power during a short period of time to the extent that the plunger is hardly moved so as to take out a current response waveform at both ends of the current detection resistance, based on which the position of the plunger is determined.
However, this prior art technique is a technique for carrying out a check when the solenoid valve is not actuated. If such check takes place while actuating the solenoid valve, a value of current passing through the coil is reduced due to series arrangement of the current detection resistance and the coil of the solenoid valve, reducing an attraction force of the coil. Thus, for obtaining a desired coil attraction force, an increase in the number of turns of the coil of the solenoid valve is needed compared with the case with no current detection resistance arranged, resulting in a problem of an increase in manufacturing cost and in size of the solenoid valve.
The present invention is made in view of the above conventional problem, and has as a first objective to achieve detection of the position of the plunger during actuation of the solenoid valve by means of a low-cost and small sized technique which requires no enhancement of an attraction force of the coil of the solenoid valve.
Moreover, in the solving technique, a second objective is to enhance the detection accuracy, and a third objective is to enhance the control responsibility of the plunger of the solenoid valve. Additionally, a fourth objective is to improve the control responsibility of the brake controllers, and thus the control quality by apply the solving technique to brake controllers which can implement ABS control or automatic braking control by carrying out PWM control for the solenoid valve.